This invention relates to stepped etalon devices and their manufacture.
Etalon devices are used widely in optical systems for optical wavelength measurement and control. They are simple interferometer devices that are structurally tuned to a given wavelength of interest. As such they are highly sensitive to temperature variations that expand or contract the etalon itself and change the refractive index. They are also highly sensitive to optical position, i.e. alignment. Both of these potential errors can be compensated for by using a stepped etalon device. The stepped etalon is actually a series of etalons with resonant wavelengths closely spaced to one another. Each of the etalons in the series is provided with an optical detector. The relative intensity of the detector outputs reflects any change in the principle optical wavelength.
These devices can be used to monitor and control, using electrical feedback, the output wavelength of an optical generator, typically a laser. They are used in a variety of applications for precise wavelength control, such as for channel stabilization in wavelength division multiplexed (WDM) systems.
The structure of a stepped etalon is relatively simple. In the basic embodiment it consists of an optically transparent body with one planar surface and one stepped surface. In principle, an equivalent function can be realized with discrete etalon devices, but integrating them as a single body allows for a common reference surface, and simplifies the system.
An etalon device can be made from any transparent material with reflective boundaries that create an optical resonant cavity. However, commonly used optical quality materials, e.g. fused quartz, have temperature coefficients of expansion (TCE) that are unacceptably large, making the control element also vulnerable to thermally induced drift. Etalon devices are also susceptible to thermo-optic effects, i.e. change in refractive index with temperature. New materials have been developed in which the thermo-mechanical material properties are matched with the thermo-optic properties so that, over a suitable temperature range, the thermo-mechanically induced drift in wavelength is compensated for by the thermo-optic wavelength shift. These materials are described and claimed in my co-pending patent application Serial No.(Ackerman et al. Case 24-11-22-35-2-7-15), filed Mar. 15, 2000. Among the materials that provide nearly offsetting thermo-optic and thermo-mechanical properties are AgCl, LilO3, CaCO3, CaWO4 and LiCaAlF6. From the standpoint of stability, performance, and manufacturability, LiCaAlF6 is presently the preferred choice.
The technique used to fabricate a stepped etalon begins with a body of optical quality material and precisely parallel sides. One side is provided with a series of steps by sequentially masking that side and etching by, e.g., reactive ion etching. Although the steps themselves are relatively shallow, some of the desirable optical quality materials, notably LiCaAlF6, are difficult to RIE process. Multiple steps in these materials require a long time to process, making these apparently simple devices costlier than desired. The alternative of wet etching lacks the precision desired for these structures.
We have developed a new thin film processing approach for the fabrication of stepped etalon devices. The process begins with an optical quality temperature compensated material, preferably LiCaAlF6, with two parallel opposing surfaces. To fabricate the steps, a multi-step process of deposition and selective RIE is used. The material deposited to form the steps is chosen for its adhesion and processing characteristics, with the optical properties acceptable but secondary. This option follows from the recognition that the manufacturing properties of the step layer can be allowed to dominate the optical properties because the layer, while thick enough to function in the stepped etalon, is still thin enough to be relatively inconsequential to the overall optical performance of the device. Steps can be formed on both surfaces of the etalon for expanded functionality.